This predicate was taking 39s on a snapshot of Facebook Fizz because it
had disjuncts like this:
43685 ~0% {1} r34 = JOIN Type::FunctionPointerIshType#f AS L WITH Type::Type::getUnspecifiedType_dispred#ff_10#join_rhs AS R ON FIRST 1 OUTPUT R.<1>
43685 ~1% {2} r35 = JOIN r34 WITH CppType::getTypeSize#ff AS R ON FIRST 1 OUTPUT R.<1>, r34.<0>
170371500 ~2% {2} r36 = JOIN r35 WITH IRType::IRSizedType#ff_10#join_rhs AS R ON FIRST 1 OUTPUT R.<1>, r35.<1>
43685 ~6% {2} r37 = JOIN r36 WITH IRType::IRFunctionAddressType#class#ff AS R ON FIRST 1 OUTPUT r36.<1>, r36.<0>
Instead of fixing the joins in `getIRTypeForPRValue` itself, I've
changed the `IRType::getByteSize` predicate such that the optimiser
knows how to join with it efficiently.
The disjunct shown above now looks like this instead:
43685 ~0% {1} r26 = JOIN Type::FunctionPointerIshType#f AS L WITH Type::Type::getUnspecifiedType_dispred#ff_10#join_rhs AS R ON FIRST 1 OUTPUT R.<1>
43685 ~1% {2} r27 = JOIN r26 WITH CppType::getTypeSize#ff AS R ON FIRST 1 OUTPUT R.<1>, r26.<0>
43685 ~6% {2} r28 = JOIN r27 WITH IRType::IRFunctionAddressType::getByteSize#ff_10#join_rhs AS R ON FIRST 1 OUTPUT r27.<1>, R.<1>
The `AliasedUse` instruction is supposed to represent future uses of aliased memory after the function returns. Since local variables from that function are no longer allocated after the function returns, the `AliasedUse` instruction should access only the set of aliased locations that does not include locals from the current stack frame.
This new instruction is the dual of the existing `AliasedDefinition` instruction. Whereas that instruction defines the contents of aliased memory before the function was called, `AliasedUse` represents the potential use of all aliased memory after the function returns. This ensures that writes to aliased memory do not appear "dead", even if there are no further reads from aliased memory within the function itself.
